Monaural masking patterns
were obtained for pure tones spaced by octaves from 250 cps to 8000 cps at
20-100 db SL on three listeners in an attempt to provide extensive data
assessing the relative importance of aural harmonies and cochlear spread of
masking tone activity in the extension of masking to frequencies above the
masking tone. The masking patterns confirm and extend the results of others, but
especially indicate that second peaks do not necessarily occur at the second
harmonic of the masking tone. The masking patterns are explained in terms 9f (1)
the activity pattern of the masking tone in the cochlea, (2) beats between
signal and masking tones, (3) aural harmonics, and (4) suppression of cochlear
response to the signal.

THE masking pattern of a
tone has been wed, for example, by Wegel and Lane and by Fletcher, to
infer the pattern of activity set up by the tone in the cochlea. The masking
pattern is not a measure of the cochlear activity but only an indicator because,
aside from the fact that psychophysical data are used to infer physiological
activity, the ear distorts loud tones, intro­ducing harmonics not present in
the stimulus. In addi­tion, the masking tone interacts with the signal tones
used to measure its masking pattern, producing beats and difference
tones.

Inferences of cochlear
activity made from masking patterns must agree with inferences made from other
psychophysical data, and especially with more direct observations of the
physiological processes inferred. Thus, the common assumption that aural
harmonics are entirely responsible for the unsymmetrical extension of masking to
high frequencies is brought into question by the demonstration by Egan and
Klumpp of the masking of aural harmonics and by physiological evidence of
asymmetry in the response of the auditory mechanism to tones as shown by the
results of Bekesy on the mechanical movement of the cochlea, of Tasaki, Davis,
and Legouix in cochlear microphonics, and of Tasaki6 in the electrical responses
of single auditory nerve fibers.

Although the results of
Wegel and Lane, which have served as the basis for most discussions of masking,
are very extensive, they are not sufficiently detailed to evaluate an
alternative mechanism for extended masking. The present experiment attempts to
provide suffi­ciently detailed masking patterns for a representative sample
of frequencies and intensities to choose between aural harmonics and cochlear
spread as the mechanisms of extended masking, and, in general, to permit a finer
analysis of the mechanisms underlying masking.

METHOD

Monaural masking patterns
were obtained for pure tones spaced by octaves from 250 cps through 8000 cps at
20--100 db sensation level (SL) on three listeners. Two of the listeners had
normal hearing, and that of the third was normal except for a 15-db dip at 1800
cps.

The signal tones were
generated by a B&6sy audiom­eter,' passed through an electronic switch
which turned them on for 200 msec and off for 200 msec without audible
transients, and then delivered to a PDR-8 ear­phone mounted in an lIX-41/AR
cushion. The Bekesy audiometer also recorded the frequency and intensity of the
signal tones during the experiment. For masking, a constant tone from another
oscillator was mixed elec­trically with the signal tones.

In determining the masking
patterns, first the abso­lute threshold was recorded starting at the low
fre­quency end of the range. Next the listener's threshold for the masking
tone was found. Then a series of masked thresholds was obtained with the masking
tone set at successively higher levels.

Masking patterns were
calculated by drawing smooth curves through the midpoints of the intensity
variations on the records and measuring the differences between absolute and
masked thresholds at 15 points per octave. The extent of the intensity
variations was minimized, and therefore the precision in determining the
threshold maximized, by adjusting rates of frequency and attenuation change.
Frequency rates of either 2 or 4 min per octave combined with attenuation rates
of 1 or 2 db per sec were used except at 100 db SL where 1 min per octave and 3
db per sec were used to minimize hearing loss from exposure to the masking
tone.

The listener's criterion of
threshold also affects the variability of the results. Variability from this
source was reduced by interrupting the signal tones and by the instructions. The
listeners were instructed to allow the intensity of the tones to increase until
they could just detect them and then to press a button which decreased their
intensity until they could no longer follow them clearly. In the masking series
the listeners were told to respond to any sounds which maintained the same time
pattern.

The listeners also reported
the character or identity of the sounds heard during the masking series. These
reports were coordinated with indications placed on the records at the time both
were made.

And finally, to compare the
results obtained with a Bekesy audiometer with those obtained by conventional
methods, such as used by Wegel and Lanes and Egan and Hake thresholds and masked
thresholds were ob­tained for one listener for fixed masked frequencies of
1050, 1100, 1200, 1300, 1400, 1500, 1600, 1700, 1800, 1900, 1950, 2050, and 2100
cps for a masking frequency of 1000 cps at 55, 60, 65, 70, 75, 80, and 85 db SL.
The equipment was essentially the same as that described in the foregoing except
for the substitution of a fixed frequency oscillator and a discrete 1-db per
step atten­uator for the B6kAsy audiometer. The listener first determined
his absolute threshold for the tonal series by the method of adjustments and
then repeated these determinations with the masking tone present at successively
higher levels.

RESULTS AND
DISCUSSION

Masking Patterns

The masking patterns are
shown in Figs. 1-6. The extent of intensity variations in the records used for
computing the masking patterns seldom exceeded 3 db except at 100 db SL where a
faster attenuation rate was used. Variation among runs and among listeners was
greater, but the average masking curves preserve the characteristic features of
the individual records.

Comparison of the curves of
Figs. 1-5 shows that 250 cps produces masking patterns somewhat different from
those of the other four frequencies. At 250 cps all the curves are relatively
smooth and regular and not sym­metrical even at the lowest masking
level.

At the other frequencies
the curves are symmetrical at 20 and 40 db SL, but dearly depart from symmetry
at 60 db and above. The asymmetry is brought about by the failure of masking to
spread further to lower fre­quencies while it increases rapidly in both
amount and extent at higher frequencies. At 60 db SL there is little or no
masking at the harmonic frequencies, but despite this lack, the masking curves
are not smooth; the ex­treme case is the sharp second peak in Fig.
4.

At 80 db there is a rapid
increase in extended masking. At this level a second peak is found in all the
curves of Figs. 2-5. It has moved closer to the second harmonic, but it does not
always coincide with this frequency although this is not altogether apparent in
Figs. 2 and 4. The migration of the second peak is most rapid between 60 and 80
db. Only at 100 db do the second peaks fall at the harmonic. At this level peaks
may also appear at the third harmonic as in Figs. 1, 3, and 4.

The 100-db curve of 4000
cps (Fig. 5) is somewhat anomalous. It shows downward spread of masking for two
octaves and a pronounced second peak at 6000 cps, a frequency which is not a
harmonic of the masking tone. The same sort of downward spread occurs for a
masking tone of 8000 cps (Fig. 6).

The masking patterns just
described are in good agreement for corresponding masking frequencies and
intensities, even to the occurrence of second peaks at nonharmonic positions,
with the results of other writers using similar methods. In general the masking
patterns are similar to those of Wegel and Lane,' except for the positions of
the second peaks. The second peak in our results emerges dearly only for masking
fre­quencies of 1000 cps and higher, and thus the only com­parable
frequency between our study and that of Wegel and Lane is 1200 cps. Their Fig. 4
shows extended masking at 60 db SL, but no second peak either at the second
harmonic or at any other frequency. On the basis of our results, a second peak
would be expected at about 1600 cps in their data but they made no observations
at this frequency.

At 80 db Wegel and Lane do
provide sufficient data to fix the second peak at the send harmonic. This is not
surprising because in our data 80 db is the level at which the migration of the
peak to the second harmonic is nearly complete., For a masking tone of 1000 cps
one of our listeners regularly showed a broad flat-topped peak centered at 2000
cps while another showed a peak at about 1800 cps on one occasion and at 1900
cps on another.

That a possible source of
difference in results is not the difference between using the Bekesy audiometer
and fixed masked frequencies is shown by Fig. 7. Here fixed masked frequencies
were used and the curves show clearly the progressive migration of the second
peak towards the harmonic as the masking tone is raised from 55 through 85 db
SL.

Fig. 1. Average masking
patterns for 250 cps based upon three Fig. 4. Average masking patterns for 2000
cps based upon two listeners. The sensation level of the masking tone is
attached to listeners. The sensation level of the masking tone is attached to
each curve. each curve.

Fig. 2. Average masking
patterns for 500 cps based upon three listeners. The sensation level of the
masking tone is attached to each curve.

Fig. 3. Average masking
patterns for 1000 cps based upon three listeners. The sensation level of the
masking tone is attached to each curve.

Fig.4 Average masking patterns
for 2000 cps based upon three listeners. The sensation level of the masking tone
is attached to each curve.

Fig. 5. Average masking
patterns for 4000 cps based upon three listeners. The sensation level of the
masking tone is attached to each curve.

Fig. 6. Masking pattern for
8000 cps at 100 db SL for a single listener.

Fig. 7. Masking of discrete
frequencies by 1000 cps for a single listener.

Auditory Characteristics of
the Signal under Mashing

Throughout the frequency
range of this experiment the signals when presented alone possess definite tonal
quality even when they fall in the atonal interval. whenever they are subjected
to an appreciable amount of masking, however, they lose their tonal quality for
the listener seeking his threshold. Since the characteristics reported in this
experiment differed somewhat from those reported by others, they are given in
the following material because they are useful in elucidating the nature of
masking. Only the predominant aspects are reported.

At masking levels up to and
including 60 db SL, the signals maintain their tonal quality everywhere except
in the vicinity of the masking frequency. In this vicinity two kinds of beats
are heard: wavering beats and fused beats or roughness. Wavering beats are heard
when the signal and the masking tone are serrated by a few cycles per second:
the sound heard consists of a loudness modulation of the masking tone, the rate
being equal to the frequency difference between the two tones. Fused beats are
heard at greater frequency differences: the wavering becomes too fast for the
fluctuations in loud­ness to be analyzed out and the tone sounds rough,
sometimes to the point that the roughness may be the predominant aspect of the
sound.

The range of wavering beats
is the same above and below the frequency of the masking tone. Fused beats are
heard for a broader frequency range above the masking tone than below. Above the
frequency of the masking tone they are heard as the masked threshold decreases.
At 20 and 90 db the transition from beats to tone pitch does hot produce a
change in slope of the masking curve. But at 50 and 60 db SL the change in sound
from beats to tone is associated with the shoulder or second peak of the masking
curve.

At higher levels of the
masking tone, the fused beats are not displaced by the tone but by beats with a
dif­ferent basic quality which are interpreted to be beats with the aural
second harmonic. There may be a fairly broad transition zone between these two
kinds of beats or there may be a quick change from the one to the other. At 70
and 80 db SL of the masking tone, the beats with the harmonic pass through the
same phases as those with the masking tone itself as the signal ap­proaches
and proceeds beyond the second harmonic, until they in turn are replaced by the
signal with tonal quality. Although these beats are heard in the vicinity of the
second harmonic, the harmonic is not audible by itself at these levels. Only at
100 db SL does the second harmonic of the masking tone become clearly
audible.

The sounds heard in the
octave between the masking tone and its harmonics, except in the immediate
vicinity of these two frequencies, have been regarded by pre­vious
workers'-2 as difference tones. However, in the present experiment difference
tones were newer identified below 80 db SL of the masking tone and even at 80 db
only when the masked threshold was near its summit close to the masking
frequency. Difference tones were heard more often when the masking tone was at
100 db SL but by no means throughout the entire range of masking. Often at this
level the opportunity for dis­tinguishing beats and difference tones was
afforded by their being present simultaneously. In such cases beats were
identified by their wavering or roughness which is similar to those attributes
heard at much lower masking levels, while the difference tones were identified
by their tonal quality which is similar to low frequency pure tones. When beats
and difference tones were heard to­gether their thresholds often were the
same, or differed by only a few decibels, so that the masked threshold was only
slightly affected by using one or the other as the criterion for the
signal.

The reason for identifying
the interactions of signal and masking tones as difference tones instead of
fused beats seems to have been twofold. First, since harmonic distortion was
thought to be occurring, it seemed plausible to assume that intermodulation
distortion was also taking place, and hence, that any sound heard in the place
of the signal tone was an intermodulation product, namely, a difference tone.
And second, from the pub­lished values'-''4 for the frequency difference
limits for beats, it seemed that these limits were exceeded when the interaction
sounds could still be heard. However, note that the published values for the
limits of beats were obtained only for the case of two tones of equal
intensities; it is apparent from the present findings that beats can be obtained
for greater frequency differences when the two tones producing them are not of
equal intensities, the most favorable condition for beats being when the second
tone is higher in frequency and just above its masked threshold. In fact, under
these con­ditions the beats can often be eliminated by raising the second
tone considerably above its masked threshold. This finding offers further
support for the identification of these sounds as beats: if they were difference
tones, then raising the intensity of the second tone would give a stronger
difference tone, not eliminate it altogether.

Mechanisms of
Masking

The masking patterns must
be accounted for in terms of the possible mechanisms of auditory masking. First,
what determines the frequency range of masking? Clearly at low masking levels
(20 and 40 db SL) the only determinant can be the cochlear activity pattern of
the masking tone. Even at 60 db SL, excluding perhaps 250 cps, where there is a
clear departure from symmetry and well-developed second peak in some
in­stances, but very little masking at the second harmonic, still the
activity of the masking tone itself must be con­sidered as the sole
determinant of the extent of the masking pattern. The same argument may be
extended to include masking at 80 and 100 db SL. The evidence of the present
study is that up to 80 db the second harmonic was not audible even when the
height of the masked threshold would seem to require the second harmonic to be
70 db above its threshold (Fig. 3, 80 db). This evidence, along with the
findings of Egan and Klumpp' which showed masking of the aural second harmonic
by the fundamental, indicates that the activ­ity pattern extends into and
beyond this region.

If the cochlear activity
pattern of the masking tone determines the extent of masking, then what
determines the amount of masking? First, in the vicinity of the masking
frequency, what may be called direct masking, the activity of the masking tone
is responsible, but the height of the masking pattern does not truly show the
amount of activity because, as Wegal and Lane' first reported and Egan and Hake"
showed in fine detail, there is interaction between the signal and the masking
tones. The range of direct masking extends from some point on the steep portion
of the masked threshold curve below the masking tone to the dip or shoulder
above the masking tone: this amounts to almost an octave in some instances, as
at 80 db SL.

At the conclusion of the
region of beats, when the pitch of the signal returns (up to 60 db SL of the
mask­ing tone), the masking pattern is thought to reflect accurately the
level of masking-tone activity in the cochlea. Even at 80 db masking level, the
masked threshold probably depends on the activity level of the masking tone,
disregarding the beats between signal tone and aural harmonic as here lowering
the threshold, since the second harmonic is masked and becomes audible only
through these beats.

Only at 100 db SL, where
the aural harmonics are audible by themselves is it thought that the harmonics
add to the amount of masking, and then the factors determining masking would be
the same as brought out in the discussion above but with respect, of course, to
the harmonic and not the fundamental.

The foregoing proposals are
compatible with what is known of the physiological activity in the cochlea and
auditory nerve. Tasaki, Davis, and Legouix have shown that low tones produce
cochlear microphonics throughout the cochlea but that with higher and higher
tones there is shrinkage of the response area towards the base of the cochlea.
When two tones are combined there is only a partial separation of their
microphonics in the turns of the cochlea. The same principles of tuning as seen
in the cochlear microphonics are manifest in the responses of single auditory
fibers. In addition, Katsuki ed al. have furnished evidence of the nature of
masking: a pure tone produces spikes in a single fiber at a characteristic rate,
but when a low tone is also pre­sented and raised in intensity, the rate
becomes changed over to that characteristic of the low masking tone. It is to be
emphasized that inhibition or reduction of re­sponse was never observed by
either Tasakia or Katsuki el al. so that the mechanism of this masking seems
only to be the overriding of the stimulus effects of the signal by the masking
tone. Responses to beats were also shown to confirm the observations of Galambos
and Davis' at the cochlear nucleus. When beats are slow, several spikes appear
during the maximum, while there is silence or reduced response during the
minimum; when the beat rate is faster, a single impulse may be evoked during
each cycle of the beats.'' Thus, the pattern of impulses indicates the beat rate
and not the characteristic frequency of the stimulus, such as is ordi­narily
the case when a single pure tone is presented.' This finding provides a
physiological foundation for the widespread occurrence of beats in our
listeners' reports.

The same upper
frequency-difference limit may thus be assumed for beats as for the preservation
of the periodicity of pure tones in the auditory nerve fiber discharge, namely,
about 2000. This supposition permits an explanation of the peculiar occurrence
in the 100-ib curve of Fig. 5, the peak at 6000 cps for a masking tone of 4000
cps: the frequency limits for beats between -1000 cps and the signal were
exceeded causing the threshold to rise, only to be again depressed as the signal
tone came within range of beats with the second harmonic (8000 cps) which is
manifestly present at this level. The possibility that the peak arose from an
actual 6000-cps tone produced by a subharmonic of the 4000­cps masking tone
interacting with either the masking tone or second harmonic to produce a
summation or difference tone must be discarded because no 6000-cps tone was
heard either alone or through beats.

Similarly, a subharmonic
cannot be invoked to explain the downward spread of masking from 4000 cps or
8000 cps at 100 db SL since no subharmonic was heard, there were no beats heard
near the subharmonic frequency, and there was no peak at this frequency. At
present, it cannot be said that this downward spread of masking is a result of
spread of the masking-tone activ­ity towards the apex of the cochlea since
such has not been observed. It is not remote masking, since this results from
detection of the envelope of the stimulus in the cochlea'8 There is just one
physiological observation of this effect, the reduction of action potentials to
a 500-cps tone pip as a 6950 tone was raised from 100 to 120 db, but this also
caused a drop in cochlear micro­phonics'8 Thus, the mechanism for this
effect is different from that of the other masking demonstrated in this paper,
and seems to involve interference or inhibition of some
sort.

CONCLUSIONS

The masking pattern of a
pure tone results primarily from the activity pattern of the tone in the
cochlea. At low intensities, the cochlear activity is confined to a local region
and the masking pattern is narrow. As the masking intensity increases, the
cochlear activity spreads only towards the base, while retaining a maxi­mum
at the locus of original response, and the masking pattern extends
unsymmetrically to high frequencies.

Aural distortion plays a
much smaller role in masking than had previously been supposed. Aural harmonics
axe not responsible for the extension of masking to the octaves above the
masking tone since they are, for the most part, masked themselves by the
fundamental tone activity. Only at 100 db SL do these harmonics emerge
sufficiently above their masked thresholds to add masking to that resulting from
the masking tone itself.

Similarly aural difference
tones also have less effect on masking patterns. They do lower the masked
thresh­old, but this again is at 100 db SL and for short ranges at 80 db
SL.

For the most part what have
previously been called difference tones are really fused beats, and it is these
fused beats that are heard in the interval above the masking tone between the
end of wavering beats and the return of the characteristic pitch of the signal.
These beats depress the threshold by amplitude modulating the masking
tone­

The mechanism of the
masking described in the fore­going results from the overlap of the masking
tone and signal in the cochlea. Another kind of masking, down­ward spread,
occurs with high frequency masking tones. This appears to result from a
different mechanism, interference or inhibition.

In a
previous study it was proposed that tonal masking arose mainly from the cochlear
activity pattern of the masking tone, modified by the formation of beats between
the signal and masking tones. The present study casts further light on these
proposed mechanisms by comparing the masking effects of pure tones of 500, 1400,
2000, and 4000 cps at 60 and 80 db SL with 1/3 octave hands of noise of equal
intensities and centered at the same frequencies. The results show that the
noise bands produce about the same amount of extended masking despite the
absence of any possible aural harmonic distortion, but greater direct masking
due to the elimination of beats. Furthermore, the noise-mashing curves join the
tone-masking curves at the second peak in the latter, providing strong
additional support for the proposed mechanisms of auditory masking.

IN a
recent study' of masking by tones it was found (1) that above the masking
frequency the masking pattern became irregular and unsymmetrical with re­spect
to the low frequency side before any appreciable masking occurred at the second
harmonic of the masking tone and (2) that even at higher masking levels where
appreciable masking occurred at the second harmonic a secondary peak in the
masking curve fell between the masking frequency and its second harmonic.

These
general findings together with others in the same study and in the work of
others seemed to require a revision of the theory of masking. 'therefore, it was
hypothesized that the masking pattern of a tone was determined primarily by two
mechanisms, the activity pattern set up by the masking tone in the cochlea and
auditory beats resulting from the superposition of the test tone upon the
masking tone in the cochlea. It was thought that the activity pattern was the
primary de­terminant of the extent (in frequency) and amount (in intensity) of
masking, and that beats, where they occurred, served to lower the amount of
masking. The hypotheses minimized the role of aural distortion (harmonics and
difference tones), for the most part relying on the extension of the activity
pattern of the masking tone toward the base of the cochlea as shown in
physiological experiments" for an explanation of the rapid advance of masking
upon high frequencies, and relying on the widespread occurrence of beats to
explain some of the details of the obtained masking patterns.

Since
direct tests of these hypotheses are not feasible, further evidence bearing upon
them must come from indirect tests. Such indirect tests can be made by using a
nonperiodic masking stimulus such as a band of noise and comparing its masking
patterns with those produced by tones. With a noise band of the same intensity
as a tone, the energy at each frequency within the band will be much lower than
that of the tone, thereby producing much less, if any, harmonic distortion.
Under these conditions, the classical view would expect no extended masking, and
the masking pattern of the noise should be symmetrical. Of course, the situation
is not that simple, being complicated by the fact that the cochlea is not a
perfect frequency analyzer, but if the noise band is broad enough to include a
few critical band widths its energy should be sufficiently spread out in the
cochlea to minimize the production of aural harmonics due to overload.

On the
other hand, the hypotheses proposed. here, attributing extended masking to
spread of cochlear ac­tivity, would still predict extended masking to occur as a
result of a noise band, and to about the same extent as for a tone at the same
overall SPL. In addition, these hypotheses predict that there would be more
masking at the frequencies in the noise band than with the single tone, but that
the noise-masking curve would join the tone-masking curve at the latter's
secondary peak due to the elimination of beats between the tonal signal and the
masking stimulus. Of course, it is expected that the noise-masking curve would
extend somewhat farther to low frequencies simply because the noise band extends
to frequencies lower than the masking tone.

Certain evidence already available encourages the ex­pectations embodied in the
preceding paragraph. Egan and Hake in determining the masking pattern of a
simple auditory stimulus, compared the masking pat­terns of a 400-cps tone and a
band of noise 90 cps in width centered at 410 cps, both at 80 db over-all SPL.
Their Fig. 7 portrays almost exactly what the present hypotheses predict. It was
thought, however, that a broad sampling of frequencies and levels should be
studied to subject these hypotheses to a more rigorous test.

PROCEDURE

The
equipment and procedure were the same as used in the previous study' except for
the noise bands which were produced by passing the output of a gas-tube noise
generator through the one-third-octave filters of a Bruel and Kjaer Type 2109
Audio Frequency Spectrometer. Thresholds and masked thresholds were obtained by
a B&6sy audiometer for two listeners with normal hear­ing; masking stimuli were
tones at 500, 1000, 2000, and 4000 cps at 60 and 80 db sensation level (SL) and
I octave bands of noise centered at the same frequencies and set to the same
over-all SPLs.

RESULTS AND DISCUSSION

Typical results for one of the subjects are portrayed in Figs. 1-8. In Figs.
1-4, which give the results at 60 db SL, the tone-masking curves are
unsymmetrical, but the noise-masking curves are all symmetrical and in addition
coincide with or approximate the tone­masking curves at the latters' shoulder or
second peak. This agreement between hypotheses and results, while gratifying,
may be due more to the width of the noise band than to the correctness of the
hypothesis because, the noise-masking curve is about equally far from the filter
characteristic curve both above and below the noise band. And since the band of
masked frequencies is so much wider than the width of the masking stimulus (even
the one-cycle-wide masking tone produces a broad peak in the masking curve), the
upper frequencies in the noise band could have pushed the masked threshold
beyond the limits of the second descending portion of the tone masking curve in
Figs. 2 and 4.

More
satisfactory for evaluating the proposed hy­potheses of masking are the results
obtained at 80 db SL, shown in Figs. 5-8. In these figures, although the
mask­ing curve slopes off below the noise band close to the filter
characteristic curve, above the band limits the extended masking curves run well
above the high frequency cutoffs. Moreover, in these figures the agree­ment
between data and hypotheses is excellent. Within the frequency limits of the
noise band there is much greater direct masking than is produced by the pure
tone; above the band limits the noise masking curve falls off rapidly,
intersecting with the tonal curve at the second peak and thereafter coinciding
with it for perhaps half an octave (in Figs. 5-7) before again diverging. The
lone exception occurs in Fig. 8 where the noise curve never does quite reach the
tone curve; this de­parture may be due to the less satisfactory dropoff of the
filter high-frequency response.

Although the general agreement is excellent, two minor features require comment.
The irregularities in the noise-masking curves and the fact that, where the tone
and noise curves of extended masking diverge, the noise curve always runs below
the tone curve.

The
irregularities in the noise-masking curves, seen as the humps at 1600 cps in
Fig. 6, 3000 cps in Fig. 7, and 6000 cps in Fig. 8, appear sufficiently often in
about the same locations that they may not be regarded as fortuitous. They have
also appeared in the data of Egan and Hake' (Fig. 2) and Bilger and Hirsh (Figs.
2 8). They do not appear at the right frequencies to be re­garded as aural
harmonic distortion, and they occur when the level of the energy per cycle of
the noise is too low to produce harmonic distortion. Thus, the possi­bility that
the extended masking is a result of harmonic distortion is ruled out. This
leaves the possibilities (1) that the hump is indicative of the activity pattern
of the noise band in the cochlea and, somewhat overlap­ping, or (2) that the
hump is a product of the interac­tion of the signal tone and masking noise
similar to the processes postulated for the second peaks in the tonal masking
curves.

Regarding the first possibility, that the hump is a function of the form of the
noise pattern in the cochlea, nothing can be said because of a lack of direct
evidence. Regarding the second possibility, the interaction be­tween tone and
noise, the change in phenomenological characteristics of the signal tone at
threshold when its frequency lies within or dose to the band limits of the noise
can be mentioned: The signal itself becomes noise­like. Similar observations
were reported by Egan and Hake" for the masking by a band of noise 90 cps in
width and centered at 410 cps. These same investigators also obtained less
direct masking within the noise band than for a broad band noise (0-1000 cps) at
the same spec­trum level. Thus, it appears that a narrow band of noise retains
some tone-like properties. Perhaps the noise band maintains a somewhat periodic
discharge in the auditory nerve fibers, a discharge modulated by the
superposition of a tone of appropriate frequency. Such a possibility was
suggested in the case of tonal masking. Again, direct evidence is lacking.

It is
not easy to explain why, at frequency regions above their coincidence, the
noise-masking curve runs below the tone-masking one. Were it not for the long
run of coincidence, it might be thought that, since much of the energy in the
noise is below the frequency of the tone, there should be less extended
masking; but then it would also be expected that the entire extended mask­ing
curve of the noise band would run below the tone. The difficulty of explanation
is further complicated by the return to coincidence toward the high-frequency
end of masking. Nevertheless, these results cannot be satisfactorily explained
even by reverting to hypothetical aural harmonics.

At
this point the discrepancy must be merely ac­knowledged as a difficulty that
requires explanation. The same result also appeared in the findings of Egan and
Hake (their
Fig.7)
in consequence of an even narrower noise (less than 2 critical bands wide) than
used in the present experiment.

It is
concluded that the present results add further support to our hypothesis that
spread of activity in the cochlea, and not aural harmonic distortion, underlies
extended masking.

Fws.
1-8. Masked thresholds for tones us noise bands. The intensity scale is
arbitrary but is the same for all figures. In each graph the lowest curve is the
absolute threshold. Of the upper curves, the continuous one is the masked
threshold for pure tone masker, the dashed one is for the noise band, and the
dash double dot gives the frequency characteristics of the filter settings used.
The filter charac­teristics are not plotted at intensities relative to the
thresholds but merely to facilitate comparison of the masked threshold with the
spectrum of noise producing it. The actual frequency limits and SPL's of the
noise are given in the legends to the individual figures.